FIG. 1 is a diagram illustrating a structure of an example of a communication network. In FIG. 1, terminal equipments 10 and 11 are respectively coupled to routing apparatuses 12 and 13. The terminal equipments 10 and 11 are end-user terminal equipments which transmit and receive Ethernet (registered trademark) frames and IP frames.
Unique MAC addresses and IP addresses are allocated to the terminal equipments 10 and 11. The MAC address becomes a transmitting source address or a destination address of the Ethernet frame, and the IP address becomes a transmitting source address or a destination address of the IP frame.
The routing apparatuses 12 and 13 form a network system having a trunk structure between the routing apparatuses 12 and 13, in order to achieve a link aggregation (LAG) or a channel redundancy.
The link aggregation couples the routing apparatuses 12 and 13 by two or more links, such as channels 14 and 15, for example, in order to form a single trunk 16, expand the band and speed up the communication. On the other hand, if a fault is generated in one of the channels 14 and 15 forming the trunk 16, the channel redundancy continues the communication via the other of the channels 14 and 15.
The links and the channels are bidirectional communications channels between two apparatuses. For example, the channel 14 illustrated in FIG. 1 enables the communication from the routing apparatus 12 to the routing apparatus 13 and the communication from the routing apparatus 13 to the routing apparatus 12.
The routing apparatuses 12 and 13 have routing tables in which routing information is registered in correspondence with the destination addresses. The routing apparatuses 12 and 13 have a function of registering the transmitting source address and an input port in correspondence with each other in the routing table when the frame is received. The routing table is provided for each input and output card which transmits the frame to and receives the frame from another apparatus (terminal equipment or another routing apparatus, etc).
The routing apparatuses 12 and 13 also have an aging function which deletes routing information (entry) corresponding to a target transmitting source address from the routing table if the frame including the target transmitting source address is not received for a predetermined time (aging time).
Next, a description will be given of the transmission and reception of frames between the terminal equipments 10 and 11.
The terminal equipment 10 transmits a user frame 17 to an input and output card 12a within the routing apparatus 12. This user frame 17 includes the destination address, the transmitting source address, and data, as illustrated in FIG. 2.
In this state, the input and output card 12a within the routing apparatus 12 refers to the routing table of the input and output card 12a using the destination address in the frame 17, and transmits the frame 17 to an input and output card 12c within the routing apparatus 12 via a switch part 12b in accordance with the corresponding routing information registered in the routing table of the input and output card 12.
The input and output card 12c within the routing apparatus 12 receives the frame 17 via the channel 14, and refers to the routing table of the input and output card 12c using the destination address in the frame 17, and transmits the frame 17 to an input and output card 13c within the routing apparatus 13 via the channel 14 in accordance with the corresponding routing information registered in the routing table of the input and output card 12c. 
The input and output card 13c within the routing apparatus 13 refers to the routing table within the input and output card 13c using the destination address in the frame 17, and transmits the frame 17 to an input and output card 13a within the routing apparatus 13 via a switch part 13b in accordance with the corresponding routing information registered in the routing table of the input and output card 13c. 
The input and output card 13a within the routing apparatus 13 refers to the routing table of the input and output card 13a using the destination address in the frame 17, and transmits the frame 17 to the terminal equipment 11 in accordance with the corresponding routing information registered in the routing table of the input and output card 13a. 
Similarly, a frame 18 transmitted from the terminal equipment 11 is transmitted to the terminal equipment 10 via the input and output cards 13a and 13d within the routing apparatus 13, the channel 15, and the input and output cards 12d and 12a within the routing apparatus 12.
The frame 17 is transferred using the channel 14, and the frame 18 is transferred using the channel 15. In other words, the link that is used for the frame transfer is different between the two even within the same trunk.
This is because, the link used to transfer the frames, amongst the plurality of links forming the trunk, is determined independently by the routing apparatus at the transmitting source, and the same link may be determined for the transfer of the frames.
A Japanese Laid-Open Patent Publication No. 2005-184595 proposes an IP packet ring network that learns the MAC address, and finds a shortest route between nodes without using a complex routing protocol.
In addition, a Japanese Laid-Open Patent Publication No. 2005-354598 proposes automatically selecting the shortest route by learning the MAC address from clockwise and counterclockwise routes from a transmitting node in a double ring type transmission apparatus.
In the communication network structure of FIG. 1, when the frame 17 is first input to the input and output card 12a within the routing apparatus 12, a flooding in which the frame is copied and transferred to the input and output cards 12c and 12d other than the input and output card 12a is carried out. Hence, the routing information is learned in the routing tables of the input and output cards 12c and 12d. 
Thereafter, a communication format in which the frame 17 is transmitting from the input and output card 12c within the routing apparatus 12 to the input and output card 13c within the routing apparatus 13 via the channel 14 is repeated. If the frame 18 is not transmitted, the routing information with respect to the terminal equipment 10 is deleted from the routing table of the input and output card 12d by the aging function at a time when the aging time is exceeded and the above routing information no longer exists in the routing table of the input and output card 12d. 
In this state, if the frame 18 with respect to the terminal equipment 10 is input to the input and output card 12d, the frame 18 is flooded to the input and output cards 12a and 12c from the input and output card 12d because the input and output card 12d no longer has the routing information with respect to the terminal equipment 10, and as a result, there was a problem in that the transmission performance within the routing apparatus 12 deteriorates.